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Argyle, AK1 Pipe

Western Australia, WA, Australia

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The Argyle, AK 1 diamond pipe is located ~75 km south of Kununurra, ~540 km SE of Darwin and 2200 km NE of Perth in far northern Western Australia (#Location: 16° 42' 47"S, 128° 23' 10"E).

The pipe is near the eastern edge of the 400 x 100 km Halls Creek Mobile Belt which comprises a deformed late Archaean to Palaeoproterozoic package of crystalline metamorphic, sedimentary and igneous rocks (including 2.5 to 1.8 Ga granites) forming the eastern margin of the stable 400 x 500 km Kimberley Block. The Kimberley Block, which is bounded to the SW by the similar King Leopold Mobile Zone, comprises a plateau of near flat-lying, 1.9 to 1.6 Ga sedimentary and volcanic rocks, underlain by a basement of crystalline igneous and metamorphic rocks, which are not exposed on the surface of the plateau, but have and indicated Archaean age (Graham et al., 1999). To the north and NW, the block is bounded by the Indian Ocean.

The pipe was emplaced within a pre-existing fault, ~7 km west of the parallel Halls Creek Fault, which forms the eastern boundary of the Halls Creek Mobile Zone (Boxer et al., 1989), and is a major NNE trending structural feature that punctuated the sedimentation and structure of the region from the Palaeoproterozoic onwards. The northern part of the Halls Creek Mobile Belt is overlain by a preserved northerly dipping package of 1500 to 500 Ma sedimentary and volcanic rocks, resistent members of which have preserved the pipe from erosion.

The AK1 pipe is a 2 km x 150 to 500 m wide lamproite intrusive to tuff covering a surface area of 47 ha. The pipe has apparently been tilted northward at an angle of ~30°. Its outline on the surface resembles the shape of a 'tadpole', with its enlarged 'head' to the north and its narrower 'tail' elongated in a SSE to SE direction. The northern 'head' section is ~600 m across and bowl shaped, with a base at around 230 m below surface. Further south the eastern contact is steeply (75° W) dipping whereas the western contact is 60° W to vertical. In the central sections, drilling has indicated the pipes is open beyond 300 m depth. To the south, a narrow pod is the result of faulting and displacement and is ~150 m wide. The pipe may be the combination of a number of vent sources and has been structurally tilted since emplacement.

The intruded sedimentary rocks, mainly sandstone, siltstone and shale, have been dated at 1158±123 Ma and 1057 ±80 Ma, while absence of clasts of the Cambrian Antrim Plateau Basalts puts an upper Cambrian limit to the pipe.The pipe rocks have yielded dates from different techniques of 1178±47, 1200 and 1238 Ma.

The AK1 Pipe is filled by diamondiferous olivine lamproite pyroclastic and reworked pyroclastic rocks and intruded by rare olivine lamproite dykes. The pyroclastics have been divided into two varieties, 'sandy tuff' and 'non-sandy tuff'. The 'sandy tuff' comprises volcanic clasts of olivine lamproite, exotic clasts of sandstone and shale. The fine grained volcanic matrix has been partially recrystallised to a hard potassium-feldspar, whilst the enclosed clasts range from blocky to highly vesicular. Dark green-brown blocky clasts, formed by rapid chilling when the hot magma hit water. Virtually all the primary minerals in the lamproite have been altered, with only phlogopite and chromite surviving. The sandy tuff in the northern section of the pipe is well bedded, with low angle cross bedding, scour and fill channels, accretionary lapilli and water escape structures. The 'non sandy tuff'comprises olivine lamproite clasts in a finer grained altered ash matrix. The lamproite dykes have been altered to either olivine phlogopite lamproite, or phlogopite olivine lamproite, and cross cut the sandy tuffs as a late stage intrusion. The margins of the pipe are shattered fragmental volcanic breccia. This marginal area contains zeolites, mica, kaolinite and clays.

The mean major element chemical composition of the Argyle lamproite in wt.% is: 44.0% SiO2; 3.40% TiO2; 5.29% Al2O3; 8.02% FeO; 0.14% MnO; 19.7% MgO; 5.27% CaO; 0.16% Na2O; 3.92% K2O; 1.25% P2O5; 4.83% H2O; and 2.69% CO2 (after Mitchell et al., 1991).

The bulk of the diamonds are found within the intact core of the volcanic pipe, as well as within some of the marginal breccia facies and maar facies, whilst the highest grades of diamonds are found in the sandy tuff, at 3 to 30 carats per tonne, with lower amounts in the other two rock types. In the northern part of the diatreme, these rocks are overlain by shales. Contact breccias are developed on the margins of the pipe, comprising blocks up to 1 m across of country rock and juvenile lamproite clasts set in a matrix of quartzose tuff. Most of the primary silicates of the lamproite have been altered.

A study of the diamonds yield a 1580 ±60 Ma age for inclusions in the diamonds and evidence they formed in both mantle eclogite and mantle peridotite and are xenocrysts in their host lamproite.

Production consists of 5% gem, 25% industrial, and the remainder termed near-gem, which are processed to extract a normally low-value polished diamond (Shigley et al., 2001). The mine produces 90 to 95% of the world's pink and red diamonds. Diamonds from the mine tend to fluoresce blue or dull grey under UV light, and blue-white under X-ray radiation (Mindat website, visited, July, 2016).

The initial ore reserve of the AK1 Pipe was:
  Proven - 61 Mt @ 6.8 Metric Carats/tonne
  Probable - 14 Mt @ 6.1 MC/t,

In its first year of operation, 1986, 29 million metric (MCM) carats of diamonds were produced.
The alluvial diamonds of Upper Smoke Creek and Limestone Creek were mined between1983 and 1985 to yield 17 MCM of diamonds.
Production in 2002 was 33.5 MCM

Reserves & resources at the end of 2002 (Rio Tinto annual report, 2002) were:
  Open Pit hard rock reserves - 42.9 Mt @ 3.2 MC/t
  Open Pit hard rock resources - 167 Mt @ 2.8 MC/t
  Alluvial resources - 42 Mt @ 0.2 MC/t.

Reserves & resources at the end of 2006 (Rio Tinto annual report, 2007) were:
  Hard rock reserves - 105.9 Mt @ 2.1 MC/t
  Hard rock resources - 75 Mt @ 2.8 MC/t
  Alluvial resources - 28 Mt @ 0.2 MC/t.

Reserves & resources at the end of 2008 (Rio Tinto annual report, 2009) were:
  Hard rock reserves - 89 Mt @ 2.1 MC/t (open pit + underground)
  Hard rock resources - 77 Mt @ 2.9 MC/t

Reserves & resources at the end of 2016 (Rio Tinto annual report, 2016), based on a nominal 0.5 mm lower cut-off size and a final re-crushing size of 6 mm, were:
  Hard rock reserves - 29 Mt @ 2.3 MC/t (open pit + underground) for 67.4 million carats of recoverable diamonds
  Hard rock resources - 15 Mt @ 3.2 MC/t to a depth of ~625 m.
Production in 2016 was 13.958 million metric carats of diamonds.
Note: Reserves are additional to resources.

Large scale open pit mining of the Argyle pipe began in late 1985. Total production to the end of 2016 was 835 Million carats of diamonds. Peak production was from 1993 to 1998 when more than 40 Mct were produced per annum. The operation is scheduled to close at the end of 2020.

For more detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2001.     Record last updated: 25/7/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Argyle

  References & Additional Information
   Selected References:
Boxer G L, Deakin A S  1990 - Argyle alluvial Diamond deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1655-1658
Boxer G L, Jaques A L  1990 - Argyle (AK1) Diamond deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 697-706
Boxer G L, Lorenz V and Smith C B  1989 - The geology and volcanology of the Argyle (AK1) lamproite diatreme, Western Australia: in   Kimberlites and related rocks, Volume 1, Their origin, Occurrence and emplacement Proc. 4th International Kimberlite Conference, Perth, WA, 1986   Geol. Soc. Aust, Spec. Publ. 14, Blackwell, Melbourne pp 140-152
Jaques A L, Haggerty S E, Lucas H and Boxer G L  1989 - Mineralogy and petrology of the Argyle (AK1) lamproite pipe, Western Australia: in   Kimberlites and Related Rocks, Volume 1, Their origin, Occurrence and Emplacement Proc. 4th International Kimberlite Conference, Perth, WA, 1986   Geol. Soc. Aust, Spec. Publ. 14, Blackwell, Melbourne pp153-169
Jaques A L, Sun S-S and Chappell B W  1989 - Geochemistry of the Argyle (AK1) lamproite pipe, Western Australia: in   Kimberlites and related rocks, Volume 1, Their origin, Occurrence and emplacement Proc. 4th International Kimberlite Conference, Perth, WA, 1986   Geol. Soc. Aust, Spec. Publ. 14, Blackwell, Melbourne pp 170-188
Skinner E M H, Smith C B, Bristow J W, Scott Smith B H and Dawson J B,  1985 - Proterozoic kimberlites and lamproites and a preliminary age for the Argyle lamproite pipe, Western Australia : in    S. Afr. J. Geol.   v88 pp 335-340


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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